Version 3.1

Radiation Protection ServiceDepartment of Wellbeing, Safety & Health

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LASER SAFETYa level 1 training course: basic competency (for new users)

30th October 2010

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How lasers work Types of laser & applications Laser classification Area designation, signs and

symbols Biological effects PPE& safe beam alignment Legislation

Contents How to use this package

The intention is that this module should be free standing overview that new laser users should be able to follow and thereby achieve an understanding of laser safety.

Please direct any queries, comments. Suggestions or requests for help here.

The package is supported by links to sources of further information (e.g. Wikipedia).

The door to the Radiation Protection Service is always open. Phone Ian (34203) or Andrew (34202)

Help

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Radiation Protection ServiceDepartment of Wellbeing, Safety & Health

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Overview: how lasers work

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What is a laser?

Let’s answer this question by considering the schematic drawing of a helium-neon (HeNe) gas-filled laser, a familiar laser in teaching and research labs.

helium-neon gas filled glass envelope

anodecathode

100% reflective

mirror

95% reflective mirror / output coupler

laser beam; λ = 632 nm

The He-Ne comprises1. An optical resonator a sealed glass envelope with a fully reflective

mirror at one end and a 95% reflective mirror at the aperture.2. A gain medium a mix of helium and neon gases.3. A power source a ~1000V electrical discharge that causes electrons

to flow through the gain medium from the cathode to the anode.

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So how does it work?

1. Electrons emitted by electric discharge collide with and excite He atoms.

2. Energy is transferred from He to Ne atoms, raising (pumping) the Ne electrons from their ground state (L1) to a high energy state (L3).

3. A population inversion is created at L3 by further pumping of ground state Ne atoms.

4. Spontaneous + stimulated emission of electrons from L3 to L2 releases 632 nm photon.

5. Electrons decay rapidly from L2 to L1.6. Because the stage from the release of electrons through to Ne-L2 is

faster than the stage from L2 to L1 the gain medium remains saturated, the population inversion is maintained, and the laser continually emits 632 nm photons.

cathode

anode

Ne-L1

L2

L3 632

nm

heatHe

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More on population inversions

Many substances will support electron transitions such as described for helium-neon.

However, for a substance to be suitable for use as a gain medium it has to be able to support a population inversion, i.e. if a medium can support more electrons in excited states than in ground states then it will be possible to pump electrons into higher states faster than the rate of spontaneous decay, and thereby achieve a continuous emission of photons.

Other examples of complex gain media include those used to fill carbon dioxide lasers (~20% CO2 + 15% N2 + ~3% H2 / He2) and the solid state YAG lasers (yttrium aluminium garnet host doped with ‘impurities’ such as neodymium, chromium, titanium).

If a population inversion can be supported, the process of stimulated emission becomes possible.

Stimulated emission is ‘why lasers are’. Laser beams are monochromatic wavelengths which oscillate between two mirrors at either end of the laser cavity. When the mirrors are set to the correct harmonic the reflected ‘waves’ become constructive, i.e. they are in phase (coherent) and the energy of the beam is summed.

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...and more on stimulated emission

(1)

(2)

(3)

(4)

(5)

(1)An excited orbital electron spontaneously decays and emits a characteristic photon [L3 L2].

(2)The passage of a photon corresponding to the energy gap L3 L2 induces the emission of a photon of the same frequency as the passing photon.

(3)The emitted photons are in phase, and a constructive standing wave develops.

(4)The wave is reflected into the gain media and stimulates further emission.

(5)In a continuous wave laser ‘light’ is emitted through the partially reflective mirror.

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Continuous wave or pulsed?

Thus far we have looked at continuous wave (CW) lasers, where the power output is continuous and is directly related to the steady pumping of the laser. Typically, power outputs from CW lasers were in the region of miliwatts to tens of watts, although technological developments are now realising kilowatts of power.

Pulsed lasers, however, deliver peak outputs of hundreds to thousands of watts in ‘trains’ of pulses where each pulse lasts a fraction of a second. Pulsing finds particular use in laser drilling and ablation, where there is sufficient energy deposition to vaporise shallow depths / small volumes of materials. By contrast, low energy femtosecond pulsing allows biochemical reactions or physical changes to be followed. Q-switching - crudely, an attenuator is fitted inside the optical cavity that

enables the power of a CW laser and releasing it in short gigawatt pulses. [High pulse energies, long pulse duration].

Modelocking - uses the time-bandwidth between oscillating standing waves to modulate the production of constructive waves at pico / femtosecond intervals.

Gain switching – electrons are pulsed into the active lasing medium, causing electron gates cycle between open & closed states stimulating photon emissions.

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Resumé

Excepting diode lasers, which work by ‘gating’ an electron flow, lasers are very simply an optical or resonant cavity with highly polished mirrors at either end, one fully reflective the other partially so (the aperture).

The cavity is filled with a lasing medium (gas, liquid / dye, or a solid matrix) that is capable of existing in a predominantly excited state when pumped.

The length of the cavity is related to the wavelength or harmonics of the emitted photons (laser ‘light’)...key is the creation of a constructive coherent standing wave.

The laser medium is pumped (energised) by a power source, which may be electric discharge, a flashlight, another laser...anything that can supply a constant energy flux.

Lasers may be pulsed in order to increase peak power or achieve short pulse duration.

Laser beams are monochromatic (or comprise a few related λ), are coherent and collimated (low beam divergence).

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Classic Spectacular Concert, photograph by Fir0002/Flagstaffotos, reproduced under GFDL licence.

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Radiation Protection ServiceDepartment of Wellbeing, Safety & Health

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Types & applications...coming up next...

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Laser types

Laser types are classed by their gain media and broadly fall into four categories.

Gas lasers: carbon dioxide, excimer, HeNe,

Solid lasers: Nd:YAG, ruby, Ti-saphire,

Liquid / dye lasers: tunable lasers using chemical dyes to select the wavelength of interest

Laser diodes: semi-conductor lasers.

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Laser applications

Applications include Raman spectrometry: monochromatic photons excite chemical bonds

or orbital electrons to induce characteristic atomic / molecular vibration effects that can be used to fingerprint molecules, investigate chemical bonding and composition, study temperature effects, and characterise materials. In addition, polarised light can be used to probe crystalline structures. Thus applications are found in the physical sciences, forensics, archaeology, process monitoring.

LIBS: another spectroscopy tool, where samples of ablated materials are formed into a plasma the plume subject to spectrometry.

Materials processing: ablation, cutting, engraving, drilling. Medicine & healthcare: surgery, dentistry, ophthalmic surgery,

cosmetic surgery, hair removal. Product development: printers, measurement, optical discs, pointers,

holography. Recreational: laser light shows. Military / police: dazzlers, weapons systems, range finding, speed

cameras.

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Laser harp used by Jean Michel Jarre, photograph by Wikipedysta:Maksymus007, reproduced under GFDL licence.

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Radiation Protection ServiceDepartment of Wellbeing, Safety & Health

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Laser classificationThe measure of danger

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Classification schemes

Lasers are classified under BS EN 60825-1:1994 or its replacement BS EN 60825-1:2007. Both standards are presented during the course of the next few slides because lasers labs are likely to have lasers and laser products that have been classified under both schemes.

The standards are available free from the British Standards Institute: enter their site through the institutional login page.

Two things to remember: laser class and, laser product classification.

The laser class is the classification of the actual laser itself, whereas the laser product is the classification of the device or instrument: example 1) a CDROM drive is a class 1 product that contains a class 3B laser that the laser has been rendered inaccessible renders the product as class 1, example 2) a confocal microscope may be fitted with a class 3B laser that is inaccessible under normal operating conditions product class 1.

Read PD IEC TR 60825-14:2004, this is the definitive document that underpins laser safety across the globe

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The ‘old’ laser classification scheme

Class Reason Power limits

Class 1Safe

Lasers are safe under reasonably foreseeable conditions.

<0.98 mW

Class 2 (visible)Safe - low power

For CW lasers protection is afforded by the blink reflex (0.25 secs).

<1.0 mW

Class 3AUsually safe - low power

An extension of Class 2, where the blink reflex protects.Using optical aids may be hazardous.

<5 mW(irradiance <25 mW-2)

Class 3BCaution - medium power

Direct intrabeam viewing is always hazardous.Spectral reflections may be hazardous.The viewing of diffuse reflections is usually safe.

<0.5 W

Class 4Warning - high power

Diffuse reflections are hazardous and may cause skin burns.Fire hazard; laser beams can drill through metal.

>0.5 W

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The revised classification scheme: low risk

Laser class Hazard Control measure

Class 1 (all wavelengths)Minimal risk

Safe. No protective measures are necessary.

Class 1M (all wavelengths)Low risk

Beam divergence ensures safe to eyes.

Do not stare into the beam.Do not re-focus the beam.Prevent viewing through binoculars, optical sights etc.Prevent beam being directed towards people.

Class 2 (visible λ only)Low risk

Protection afforded by the blink (aversion) reflex.

Class 2M (visible λ only)Low risk

Safe under normal operational conditions. May be unsafe if magnified viewing instruments used.

Prevent direct viewing of the beam.Use a ‘beam stop’ to terminate the beam.

Class 3R (all wavelengths)Low risk

Safe under normal operational conditions.

Prevent direct eye exposure to the beam.

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The revised classification scheme: high risk

Laser class Hazard Control measures

Class 3B (all wavelengths)CautionModerate risk

Laser beams have a level of emission that is harmful to the eye and potentially harmful to the skin.Spectral reflections may be harmful to the eye.

Prevent intrabeam viewing.Prevent eye and skin exposure to spectral reflections.Prevent laser beams from leaving the optical bench.Terminate all beams.

Class 4 (all wavelengths)WarningHigh risk

Laser beams have a level of emission that is always harmful to the eyes and skin.Spectral and diffuse reflections are always harmful to the eye.Diffuse reflection may be harmful to the skin.Risk of fire or fumes.

Prevent eye and skin exposure to primary laser emissions and to diffuse or scattered radiation.Prevent against laser interaction hazards.Ensure barriers and screens likely to be struck by beams will not ignite or melt.

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Warning signs

CAUTION – CLASS 3B LASER RADIATION

WHEN OPENAVOID EXPOSURE TO

THE BEAM

CAUTION – CLASS 4 LASER RADIATION

WHEN OPENAVOID EYE OR SKIN

EXPOSURE TO DIRECT OR SCATTERED

RADIATION

Areas and equipment should be designated according to the risk they pose...

CAUTION means low risk which if not avoided could result in minor / moderate injury.WARNING serious injury / death.

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Radiation Protection ServiceDepartment of Wellbeing, Safety & Health

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Biological EffectsLaser-human interactions

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Tissue damage threshold – wavelength (1/2)

The extent of tissue damage is strongly related to the energy deposited in a volume of tissue, although wavelength is important in that ultraviolet, visible and infra red photons penetrate to different depths of tissue.

UVC UVA Visible IR

This means that Ultraviolet radiations effect surface tissues, such as the epidermis or cornea of the eye. Infra red radiations penetrate into deeper tissue, e.g. Structures in the subcutaneous layer or the retina of the eye. UV may burn the hairs of the skin, but IR will damage the root.

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Tissue damage threshold – intensity (2/2)

Green lasers of different intensities, photograph by Gonioul, reproduced under GFDL licence.

However, the energy deposited will determine the extent or amount of damage; a 50 mW Class 3B UVC laser beam may cause the skin to tingle, but a 500mW beam will burn.

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Damage mechanisms (1/2)

Photochemical damageThe deposition of relatively low energies by absorbed photons causes the chemical excitation of irradiated molecules. Damage is usually reversible.e.g. Exposure to diffuse reflections from a Class 4 UV excimer laser may cause skin erythema or photokeratitis (inflammation of the superficial cells of the cornea).

Thermal damageThe absorption of radiant energy causes the vibration of molecules and localised heating, possibly leading to the coagulation of proteins. Damage at higher intensities is irreversible.e.g. Exposure to specular reflections from a Class 3B diode IR laser may cause deep skin burns , hair loss – follicle damage or enzyme denaturation in skin glands.

Thermo-acoustic damageHigh irradiances delivered over short time periods, such as from pulsed lasers, cause rapid thermal expansion of tissue and vapourisation of cellular components...explosively!e.g. Exposure to specular reflections from a Class 4 visible laser may cause deep tissue burns or puncture the retina

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Damage mechanisms (2/2)

Damage to the retina of the eye following exposure to a 40 mW HeNe laser

Thermal damage: localised coagulation marks on the retina

Thermo-acoustic damage: internal bleeding from the retina

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Absorption of light by the eye: ultraviolet

ArF excimer (193 nm)

XeCl excimer(308 nm)

Cadmium vapour

(325 nm)

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Absorption of light by the eye: visible

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Absorption of light by the eye: infrared

Nd:YAG(1.06 μm)

HF(2.9 μm)

CO2

(10.6 μm)

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Beams in fog + car windshield, photograph by Jeff Keyzer, reproduced under GFDL licence.

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Radiation Protection ServiceDepartment of Wellbeing, Safety & Health

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PPE & safe beam alignmentShould I wear laser goggles?

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PPE, a few don’ts and a do

What protective equipment, and when to wear it, should be determined by risk assessment.

If the laser process includes hot work , chemicals, dirt you may need to wear thermal gloves, latex / nitryl gloves, lab coats, etc.

Aligning lasers and setting optics, basically most optical bench activities, are impossible with gloves. You will need instruction from a competent person on how to carry out these activities safely.

Don’t Wear jewellery, watches, bangles, dangling neck chains etc.

Laser beams can be reflected off the laser bench by shiny objects.

Wear loose clothing or ties when leaning over lasers. They can catch and misalign optics or catch fire if beam energies are high enough.

Do Protect the beam path and optics at all times.

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PPE: eyewear – the problem

Reliance on laser goggles is dangerous! Laser goggles only work for specific wavelengths and for specific

energies.

Assume a lab has several lasers operating. Any scattered beams from one laser e.g. an IR laser, could cross the lab and the goggles worn by the UV laser user would not protect them.

Also, a laserist may wear goggles that protect them. But a colleague nearby without goggles will not be protected.

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PPE: eyewear – the solution

Work MUST be carried out in a such a way that it is not possible for stray beams or reflections to leave the optical bench. This is the only way to maintain a safe environment.

Only wear eye protection if there is a non-trivial risk of injury from accidental exposure.

If goggles are to be worn then everyone in the lab must wear a pair.

Check that: the optical density (OD) will reduce

laser energy below the MPE (safe level),

the goggles are CE marked and are marked with wavelength(s) they will absorb,

the goggles fully enclose the eyes, the goggles are in good condition,

clean and fit properly.

Read PD IEC TR 60825-14:2004 pp44-45

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Beam alignment

Most optical related injuries occur during beam alignment, and the groups at most risk are novices and the very experienced.

This is a simple checklist to help you make the right decisions and stay safe. Can you use a low power laser mounted in tandem? If not, can you align with the laser turned to low power, e.g. <25 mW? If not, is it practicable to use a camera, remote tool or viewing aid? If not, is it practicable to wear goggles and will you be able to see (some goggles cut out visible light.

Are you going to use burn card, phosphorescent card, or black card?

Have you been shown how to ‘lead’ laser beams through optical arrays? And, have you practised with low power lasers?

Remember that optics can create multiple reflections. Do you know where the paths of all reflections will lie? Be aware of prisms moving beams in unexpected directions. Be wary of reflective and shiny metal surfaces. Always terminate beams at the end of each alignment phase.

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Laser tuning, reproduced by permission of the University of

Leeds.

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Radiation Protection ServiceDepartment of Wellbeing, Safety & Health

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Legislation, rules & risk assessment

Safety management and organisation

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The statutory framework!

Health and Safety at Work etc Act 1974 Section 2 UoL has a duty to look after you, make sure equipment is safe, risks are assessed and provide information and training. Section 7 You have a duty to co-operate, not to misuse or interfere with anything, and to look after yourself and not endanger your colleagues.

Management of Health and Safety at Work Regulations 1999 Reg 3 Employer must make risk assessments and identify control measures.Reg 5 Employer must plan, organize, control, monitor and review. Reg 7 Appoint competent persons and give them time and the means to assist. Regs 8 & 9 Emergency plans and medical arrangements. Reg 10 Give information Reg 13 Ensure staff are capable of performing tasks and given training. Reg 14 Employees must follow procedures. Regs 16-18 Arrangements for pregnant workers.

Control of Artificial Optical Radiation at Work Regulations 2010 Same as the above

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What UoL has done

Competent persons Faculties / Schools have appointed Laser Safety Officers who are competent and trained to a high standard (Health Protection Agency / Loughborough University Laser safety Management Course). Your LSO is there to help you.

Revitalized its safe organization Health & Safety Policy identifies duties associated with roles e.g. Dean, HoS, Managers, Competent Persons. Read the policy here.

Risk assessment An Excel based risk assessment procedure.

Information Local rules (instructions on how to work safely with lasers) and guidance documents are hosted on the VLE. To obtain access you must register for a permit.

Permit system To work with lasers you must have a valid reason and apply for a permit. LSOs will be able to download you an application form from the VLE.

Design assessment & critical examination procedure All equipment should be examined by the Radiation Protection Service before being commissioned for use.

Laser safety will be reviewed during the academic year 2010-2011.

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Safety management at UoL

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How to make a risk assessment

Firstly, who should make the RA. Legislation says the employer. We say yes, but it is in your own interests to do this with your LSO.

1. Consider normal operation and reasonably foreseeable conditions – no need to look at the weird and whacky.

2. Summarise : process, optical, installations (gases, coolants, electrical supply, etc.).

3. Do you have documentation, manuals data on the laser (Class, power, wavelengths, pulse duration, peak energy, repetition rate).

4. Assess risk to those exposed (laserists, cleaners, visitors).

5. Identify the MAJOR hazards...not every tiddly trifling risk.

6. Determine the risks to those who may be exposed.

7. Write it down and say when control measures will be put in place.

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Please...

common sense, common safety

Lord Young of Graffham is your friend!